Conventionally, as a fuel supply system of an internal combustion engine, such as a gasoline engine or a diesel engine, a fuel supply system of an in-cylinder injection type that includes: a high-pressure pump for increasing pressure of low-pressure fuel that is pumped from a fuel tank to be high pressure; and a pressure accumulator chamber for storing high-pressure fuel that is pressure-fed from the high-pressure pump and that directly injects the high-pressure fuel in the pressure accumulator chamber from a fuel injection valve to inside of a cylinder of the internal combustion engine has been known. In addition, as the above high-pressure pump, a high-pressure pump that includes: a plunger that reciprocates within the cylinder; a pressurizing chamber into which the fuel from a low-pressure side is introduced; and a control valve of an electromagnetic drive type that adjusts a returning amount of the fuel introduced into the pressurizing chamber has been known.
As one example of the above high-pressure pump, the plunger is connected to a rotational shaft of an output shaft (a crankshaft) of the internal combustion engine, reciprocates within the cylinder when the rotational shaft rotates along with rotation of the crankshaft, and thus can change a volume of the pressurizing chamber. The control valve is an electromagnetic valve of a constantly open type, for example, and permits introduction of the fuel from a low-pressure side passage into the pressurizing chamber when a valve body is held at a valve opening position by a spring during non-energization of a solenoid coil. On the other hand, during energization of the coil, the valve body is displaced to a valve closing position by an electromagnetic force thereof and blocks the introduction of the fuel into the pressurizing chamber. In a state where the valve body of the control valve is at the valve opening position in a volume reduction stroke of the pressurizing chamber, a surplus of the fuel is returned from the pressurizing chamber to the low-pressure side in conjunction with movement of the plunger. Thereafter, when the valve body is controlled to be at the valve closing position by the energization of the coil, the fuel in the pressurizing chamber is pressurized by the plunger and discharged to a high-pressure side. In this way, discharge amount control of the high-pressure pump is executed.
During actuation of the control valve, collision sound may be produced when the valve body collides with a movement limiting member (a stopper), and the sound may give an occupant a sense of discomfort. In Patent Literature 1, various methods for reducing the collision sound between the valve body and the stopper in the discharge amount control of the high-pressure pump by the control valve are described. In Patent Literature 1, when the valve body moves to the valve closing position, the coil is energized at a minimum current value that is required to completely close the valve body. In this way, a time spent by the valve body to move to the valve closing position is extended, and a collision speed of the valve body with the stopper is reduced. Thereby, the collision sound is reduced.
In addition, in Patent Literature 1, in order to determine the above minimum current value, actual fuel pressure and target fuel pressure of the pressure accumulator chamber are compared, and the above minimum current value is determined on the basis of a current value at which a deviation of the actual fuel pressure from the target fuel pressure exceeds a threshold. In other words, when it is estimated that the current value applied to the coil is reduced and the actual fuel pressure of the pressure accumulator chamber falls below a lower limit value, it is estimated that complete closing of the control valve is not guaranteed. In addition, when the control valve is not completely closed, it is estimated that a fuel supply of the high-pressure pump is at least limited to such a degree that sufficiently high pressure can no longer be generated in the pressure accumulator chamber. In view of the above, in Patent Literature 1, the above minimum current value is determined on the basis of the current value at which the deviation of the actual fuel pressure from the target fuel pressure exceeds the threshold.
However, in the high-pressure pump, due to an individual difference or an environmental change, a variation in a fuel discharge amount with respect to the current value that is applied to the coil may be generated, and due to this variation, the fuel discharge amount may be increased or reduced from what is assumed. For this reason, when the actual fuel pressure and the target fuel pressure are compared and it is determined on the basis of a comparison result whether the fuel is discharged from the high-pressure pump (whether the pump is actuated), a relationship between the current value applied to the coil and an actuation state of the high-pressure pump at the current value may not accurately be comprehended.